The present invention relates to a method for the preparation of a composite membrane of silicon carbide and silicon nitride used for X-ray lithography. More particularly, the invention relates to a method for the preparation of a silicon carbide-silicon nitride composite membrane used for X-ray lithography having an outstandingly high transparency and free from the changes in the internal stress even by irradiation with high-energy beams such as high-energy electron beams and synchrotron radiations.
Along with the trend in recent years toward finer and finer patterning in the manufacture of semiconductor devices, the technology of X-ray lithography is very promising and highlighted as a patterning process in the near future in place of the conventional photolithography. Similarly to the photomasks used in the photolithography, an X-ray-transmitting membrane, referred to as an X-ray membrane hereinafter, is used as a mask in the X-ray lithography to serve as a substrate for the X-ray-absorbing patterning materials. An X-ray membrane must satisfy several requirements in order that the X-ray lithography by using the same can be performed successfully including, for example, that the membrane has a smooth surface without defects or pinholes and has a high mechanical strengths suitable for practical use in an industrial manufacturing process, that the membrane has a high light transmission which is essential in order to facilitate alignment of the mask with high precision, that the membrane has high resistance against the attack of various chemicals and moisture or water not to be damaged in the steps of etching and washing, that the membrane can withstand irradiation with high-energy beams such as high-energy electron beams and synchrotron radiations not to cause significant changes in the performance, and so on.
Various materials have been proposed for the X-ray membranes to comply with the above mentioned requirements including boron-doped silicon Si, silicon nitride Si.sub.3 N.sub.4, silicon carbide SiC and the like, of which silicon carbide is accepted as the most promising material in respect of the high Young's modulus to withstand irradiation with high-energy beams.
The X-ray membranes of silicon carbide or silicon nitride are prepared by utilizing the process of chemical vapor deposition (CVD) of the material on a substrate such as silicon followed by removal of the substrate material. A problem in the CVD method is that, since the film-forming material to be deposted on the substrate surface is formed by the chemical decomposition of the gaseous starting material or materials, undesirable extraneous species formed by the decomposition are unavoidably taken into the deposition of the film on the substrate surface to act as impurities in the membrane which cause several disadvantages. For example, the impurities may be readily dissipated by the irradiation with high-energy beams so as to cause occurrence of strain, changes in the stress, decrease in the mechanical strengths, decrease in the optical transparency and so on in the membrane. Further, it is of course that such impurities may result in the occurrence of pinholes and nodules on the surface of the membrane greatly affecting the quality of the membrane.
Alternatively, membranes of silicon carbide or silicon nitride can be prepared also by utilizing the method of sputtering disclosed, for example, in Japanese Patent Kokai 63-315768. Despite the advantages of the sputtering method that the content of impurities is relatively low in the membrane and the membrane is free from pinholes and nodules, the method is not free from the problems that the membrane of silicon carbide or silicon nitride is subject to the appearance of strain or distortion under irradiation with high-energy beams in a large dose as a consequence of the high amorphousness of the membrane. In addition, the membrane of silicon carbide or silicon nitride prepared by utilizing the sputtering method usually has a relatively low transmission of light which sometimes cannot exceed 25 to 30% at a wavelength of 633 nm when the membrane has a thickness of 1.0 .mu.m. The inventors previously have developed a composite X-ray membrane composed of silicon carbide and silicon nitride which is superior to the X-ray membrane of silicon carbide alone in many respects although the composite membrane is still not quite satisfactory in respect of the resistance against high energy-beam irradiation.